The invention relates to a dryer fabric for use in a gas heated dryer section of a papermaking machine.
A conventional papermaking machine typically includes a forming section, a press section, and a dryer section wherein the pulp is progressively formed and dried into paper. The different papermaking machine sections require different kinds of papermaking fabric to support and carry the pulp or paper through the particular section. In the dryer section of the papermaking machine, the ability of the dryer fabric to handle water and heat on the machine is of the utmost importance. Typically, fabrics in the dryer section have included woven fabrics or spiral fabrics. These fabrics must be capable of withstanding high temperatures and steam without breaking down. The dryer section includes a number of heated rollers or cylinders. One of the functions of the dryer fabric is to hold the wet paper against the heated cylinder to achieve the dryness desired.
Spiral fabrics which include elongated strips of spirals intermeshing and joined together with pintles are utilized in the dryer section of papermaking machines. For example, spiral papermaking fabrics are disclosed in U.S. Pat. Nos. 4,346,138, 4,381,612, 4,392,902, 4,490,925, and 4,755,420.
Woven fabrics used in the dryer section are generally two-ply fabrics with a low warp and weft count. This is necessary in order to achieve a sufficient openness for proper drainage.
The heated cylinders of the conventional dryer section are typically heated by steam introduced into the interior of the cylinders. The operating temperatures are in the range of 350.degree. F.
Recent developments in paper making machinery have introduced in the dryer section gas heated cylinders. The advantages of gas heated cylinders over the conventional steam heated cylinders are many.
Gas heat enables the cylinders to be heated to as much as 700.degree. F. The additional heat makes it possible for fewer cylinders to perform the dying operation. As many as 80% fewer cylinders may be used; i.e. from the normal 40-80 cylinders to as few as 8-20. This allows for an increase in production and requires less dryer fabric.
The predominant material for dryer fabrics has been polyester. Unfortunately, polyester melts at 500.degree. F. Fiberglass monofilaments wrapped with acrylic yarns have also been used to form dryer fabrics. They also, to a large extent, have been found to be unsatisfactory because the acrylic cannot withstand the higher temperatures.
Monofilaments formed from polyphenylene sulphide (PPS) or a blend of PPS and polyamide 66 have also been used to form dryer fabrics. See U.S. Pat. No. 4,755,420. These monofilaments also have the draw back that they cannot withstand the higher temperatures.
U.S. Pat. No. 4,359,501 recognizes the hydrolysis resistance qualities of polyetheretherkotone monofilaments operating at elevated temperatures of up to 500.degree. F. This is a maximum upper temperature for operating with the usual steam heated dryer operation. The patent is restricted to a woven dryer fabric which has a maximum permeability of 600 CFM (cubic feet of air per minute). Above 600 CFM woven fabrics become extremely unstable and sleazy and are not suitable as dryer fabrics. Woven fabrics in the range of 600 CFM do not present a smooth support surface and have a tendency to mark the paper product.
It is an object of this invention to provide a dryer section of no more than twenty cylinders with a continuous coil dryer fabric having superior degradation properties.
It is an object of this invention is to produce a coil dryer fabric with superior degradation properties and superior stability properties when exposed to prolonged use under temperatures in the range of 500.degree. F. to 700.degree. F.
Another object of this invention is to produce a coil dryer fabric using monofilaments having superior toughness and sufficient flexibility so as to not sustain degradation during a drying operation at temperatures over 500.degree. F.
Another object of this invention is to produce a coil dryer fabric for use with a gas heated dryer section having no more than twenty cylinders.
Another object of this invention is to produce a coil dryer fabric of PEEK monofilaments shaped to have a rectangular cross-section and a permeability of between 600-1000 CFM.
Another object of this invention is to produce a coil dryer fabric from PEEK monofilaments shaped to have a circular cross-section and a permeability of between 600-1000 CFM.
Another object of this invention is to produce a coil dryer fabric in which the pintle forming monofilament of PEEK is configured.